Bulletin of the American Physical Society
2009 APS April Meeting
Volume 54, Number 4
Saturday–Tuesday, May 2–5, 2009; Denver, Colorado
Session L4: Nucleon Microscopy |
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Sponsoring Units: DNP Chair: Kees De Jager, Thomas Jefferson National National Accelerator Facility Room: Plaza F |
Sunday, May 3, 2009 3:30PM - 4:06PM |
L4.00001: The Charge Form Factor of the Neutron Invited Speaker: I will present the charge distribution physics, experimental method and results of the recent measurement performed at TJNAF for 4-momentum transfer up to 3.5~GeV$^2$. The implications to GPDs models, flavor and isospin formfactors, as well as for the impact parameter dependent charge distribution in the neutron will be considered. [Preview Abstract] |
Sunday, May 3, 2009 4:06PM - 4:42PM |
L4.00002: The Elastic Electromagnetic Form Factors of the Proton Invited Speaker: In many senses, the internal structure of the nucleon is the defining problem of QCD, the fundamental theory of the strong interaction. The internal structure of the nucleon defines its mass, spin, and its interactions. The nucleon is the fundamental building block of the nucleus, and indeed it is the residual nucleon-nucleon interaction that governs all nuclear structure, in much the same way that residual interactions between atoms governs molecular structure. As such, a full and detailed quantitative understanding of the internal structure of the nucleon is a necessary precursor to extending our understanding of nuclear physics. A fundamental test of the QCD in the confinement region is the electromagnetic structure of the nucleon. In particular, measurements of the elastic electric and magnetic form factors of the proton, $G_{Ep}$ and $G_{Mp}$, respectively, at large momentum transfer, $Q^2$, shed new light on its internal nonperturbative structure. The ratio, $R_p = \mu_p G_{Ep}/G_{Mp}$, where $\mu_p$ is the proton magnetic moment, has been measured extensively over the last decade at the Jefferson Laboratory, using the polarization transfer method, where one measures $R_p$ directly by measuring the ratio of transverse to longitudinal polarizations of the recoiling proton in elastic electron-proton scattering. These data have revealed that the ratio decreases approximately linearly with increasing $Q^2$ above a $Q^2$ $\sim$ 1~GeV$^2$. The polarization transfer results are of unprecedented high precision and accuracy, due in large part to the small systematic uncertainties associated with the experimental technique. Most recently, the Gep-III Experiment was completed in June of 2008 in Hall C at Jefferson Laboratory. It extends the Q$^2$-range from 5.6 to 8.54 GeV$^2$. In this presentation, I will review the status of the proton elastic electromagnetic form factor data, including the latest results from the Gep-III experiment, and discuss a number of theoretical approaches to describing nucleon form factors. [Preview Abstract] |
Sunday, May 3, 2009 4:42PM - 5:18PM |
L4.00003: The Proton in the Nuclear Medium Invited Speaker: Whether the nucleon changes its fundamental properties while embedded in nuclear medium has been a long-standing question in nuclear physics. Nucleons are composite objects of quarks and gluons, the degrees of freedom of the Quantum Chromodynamics (QCD) Lagrangian. Yet, because of the highly non-perturbative nature of the phenomena in the confinement region, there are no available calculations for nuclei within the QCD framework. Conventionally, nuclei are effectively and well described as clusters of nucleons held together by a strong, long-range force mediated by meson exchange. In the effective nuclear field theory, the interaction of an electromagnetic probe with a nucleon inside the nucleus is described by taking into account the presence of the nuclear medium. Conventional nuclear medium effects such as the nucleon being off-shell, meson-exchange currents, isobar configurations, and final-state interactions need to be incorporated as corrections to the impulse approximation picture. In this context, the question arises whether in the nuclear current operator free or nuclear medium modified form factors should be used. What are the effects of the nuclear medium on the sub-nucleon structure? Polarization transfer in quasi-elastic nucleon knockout is sensitive to the properties of the nucleon in the nuclear medium. In our recently completed experiment E03-104 at Jefferson Lab in Hall A we measured the proton recoil polarization in the $^4$He($\vec e,e^\prime \vec p\,$)$^3$H reaction at a $Q^2$ of 0.8 (GeV/$c$)$^2$ and 1.3 (GeV/$c$)$^2$ with unprecedented precision. These data complement earlier data between 0.4 and 2.6 (GeV/$c$)$^2$ from both Mainz and Jefferson Lab. The measured ratio of polarization-transfer coefficients differs from a fully relativistic calculation, and is well described by either the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model or strong charge-exchange final-state interactions. The measured induced polarizations agree well with the fully relativistic calculation and indicate that these strong final-state interactions may not be applicable. [Preview Abstract] |
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